The invention relates to the formation of a layer of unstrained crystalline material intended for electronics, optics or optronics applications, and more particularly the invention relates to the production of a structure comprising the layer.
A layer is termed “strained” herein when its crystalline structure is strained elastically under tension or in compression, with the result that its lattice parameter is substantially different from the nominal lattice parameter of its constituent material, the nominal lattice parameter of a material being its lattice parameter when in its bulk form and in its monocrystalline condition. In contrast, a layer is termed “completely relaxed” or “unstrained” if the lattice parameter of its constituent crystalline material is substantially identical to its nominal lattice parameter. Thus, an unstrained layer has a stable crystallographic structure when it is subjected to external stresses such as thermal stresses. It is possible to find such unstrained layers directly in bulk wafers, but the fabrication of bulk material is limited to a small number of materials such as silicon, AsGa, germanium, sapphire and a few others.
To produce unstrained layers of other types of materials, it is known to form such a layer in the same wafer on a bulk substrate constituted by another crystalline material by interposing a buffer layer between the substrate and the layer. In such a configuration, a “buffer layer” is understood to be a transition layer that adapts the lattice parameter of the layer formed with that of the substrate. Such a buffer layer may thus have a composition which varies gradually with depth, the gradual variation in the components of the buffer layer then being directly associated with a gradual variation in its lattice parameter between the respective lattice parameters of the substrate and of the layer formed. An example which can be mentioned is the formation of a layer of SiGe from a bulk silicon substrate via a SiGe buffer layer having a germanium composition that increases progressively between the substrate and the SiGe layer. The “buffer layer” can thus be used to produce unstrained layers constituted by materials which do not exist or rarely exist in the bulk form.
Further, it may be advantageous to integrate the unstrained layers into semiconductor-on-insulator structures, with the resulting structure then comprising the unstrained layer on an electrically insulating layer, wherein the insulating material is comprised of a bulk substrate (such as a glass substrate, for example) or constituting a relatively thick layer (such as a layer of SiO2 or Si3N4) interposed between the unstrained layer and a subjacent bulk substrate. Such semiconductor-on-insulator structures have better electrical and/or optical properties compared with bulk structures, thereby improving the performance of components produced in the unstrained layer.
A semiconductor-on-insulator layer is typically produced by layer transfer from a donor wafer to a receiver wafer, with the donor wafer possibly being a bulk or composite material, such as a wafer comprising a buffer layer and a sub-epitaxial unstrained layer. The layer transfer techniques generally comprise a wafer bonding step, with the donor wafer being bonded with the receiver wafer, then a step of lifting off or removing part of the donor wafer to leave only the transferred layer on the receiver wafer. The donor wafer may instead be reduced by chemical-mechanical attack of the back portion (for example by polishing, grinding, chemical-mechanical planarization, chemical etching, selective chemical etching, etc) using techniques known as “polish-back” and/or “etch-back.” It also can be removed by splitting or detaching the donor wafer after atomic implantation at the zone which is to be cut, using a technique termed SMART-CUT®. This technique is generally known to the skilled artisan (by way of example, reference may be made to the work entitled “Silicon-on-Insulator Technology: Materials to VLSI, 2nd Edition” by Jean-Pierre Colinge, Kluwer Academic Publishers, pp 50 and 51).
Various examples of these techniques have been described. A technique for transferring a layer of unstrained SiGe from a composite donor wafer comprising a buffer layer using an etch-back technique is described in International patent application WO01/99169. However, this unstrained layer transfer technique is lengthy, expensive, and results in the loss of the donor wafer following transfer.
WO02/27783 describes a SMART-CUT® technique for transferring an unstrained SiGe layer. That technique allows the lifted portion of the donor wafer to be recovered for possible recycling. However, the layer transfer cannot ensure perfect homogeneity of its thickness, and an additional polishing step has to be carried out to increase the short range homogeneity. The polishing step reduces the long range homogeneity (i.e., the homogeneity measured between the center and edges of the wafer).
WO02/15244 and WO04/006327 describe a method of transferring a layer of unstrained SiGe from a donor wafer that includes a layer of strained Si between the buffer layer and the unstrained SiGe layer. The method consists in transferring the unstrained SiGe layer and the strained Si layer onto a receiver wafer, and then selectively removing the strained Si as opposed to the unstrained SiGe layer. That method produces a final layer of unstrained SiGe having short and long range homogeneity using the SMART-CUT® technique and without having to carry out final polishing. During transfer of the strained Si layer, however, heat treatments are applied, and this may cause diffusion of germanium into the strained Si layer, thus reducing the selectivity between the strained Si and the unstrained SiGe which is necessary for final chemical etching. Given that a strained Si layer is thin, this loss of selectivity may lead to the unstrained SiGe layer being attacked, and thus to a reduction in its quality and in its short range thickness homogeneity.
Thus, improvements in these existing procedures are desired.